Thanks for posting your query over Ansys Learning Forum.
I would like you to refer HFSS help PDF section titled 'SBR Workflow'. The detailed steps you will find in documentation.
However, I will explain in short why fields calculator is not avaialble to use in SBR simulations. The SBR simulations launches rays from source antenna towards scattering geometry which may be metallic or dieletric. The launched rays emitted from source are ignoed if they escape without hitting any surface of scattering geometry. The launched rays emitted from the source if hits metallic surfaces will get reflected, where as rays that hit penetrable dieletric surfaces, get reflected and transmitted. At ray hit point, incident, reflected and transmitted fields are computed based on material properites if the interface/surface. These fields and PO approximations are used to compute equivalent surface currents at ray hit points only. In other words, based on ray tracing principle, SBR paints equivalent currents on surfaces of scatterning geometry. These currents radiates as secondary sources to compute Rx gain or coupling at receiver. Since, there are no fields computation inside dieletric objects, you cannot use field calculator in SBR+.
Hope that this answer helps.